A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiological based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or measuring the level of the patient's physical activity. These are labeled "VVIR" for a single chamber version or "DDDR" for a dual chamber version.
The complexity of modern pacemakers, the occurrence of rare device failures, or, more commonly, physiologic changes, and device variables or drift dictate the need for numerous programmable parameters accessible noninvasively via an externally operated programmer. The need to assess system performance or troubleshoot the patient, device and/or lead system in an acute, clinical setting or long-term, while the patient is ambulatory, is increasing.
Ambulatory EKG monitoring is the most effective way of determining satisfactory pacemaker or cardioverter/defibrillator function. In the presence of a malfunction that has occurred or has been provoked by daily activity, there is no technique that provides better accuracy for determination of the state of the function of the implanted device and its interaction with the patient. Passive EKG monitoring with provocative testing, can frequently detect a malfunction of the implanted device and is the basic technique utilized today. Additionally, the storage of data in counters or, more recently, rate or trend histograms, and the use of telemetry markers to indicate device function may aid in the diagnosis of malfunction or may allow optimization of device performance. However, it is often impossible in an acute, clinical setting to duplicate specific daily events and activities, thus many problems are unresolved. Additionally, it is impossible to record for prolonged periods of time and even for short periods of time with adequate resolution. Lastly, event counters and histograms do not provide the temporal relationship between events to enable diagnosis of transiently occurring problems.
Episodic events such as transiently brief runs of pacemaker mediated tachycardia (PMT), supraventricular tachycardia (SVT) or syncope can be detected as a source of clinically significant symptoms by ambulatory monitoring and rarely occur during clinical passive EKG evaluation. Events related to daily activity, electromagnetic interference (EMI), loss of capture via specific body position or activity, and the effect of sleep or activity on patient/device interaction may be readily demonstrated by ambulatory monitoring.
However, ambulatory "holter" monitoring typically entails attaching tape-on electrodes to a patient and monitoring the surface EKG via a tape recorder or integrated circuit (IC) memory recorder worn on the patient's belt for a 24-hour period. This "strapped on" device causes patient discomfort and limits activity. If the transient event is not captured, the trial must be repeated or the troubleshooting process must be curtailed or changed to a trial and error method for problem resolution. Additionally, the evaluation of the 24 hours of stored data is a time intensive and expensive process. Lastly, standard holter monitoring has no capability for determining device function simultaneous with the stored EKG artifacts.
Devices have been proposed for the electronic storage and transmission of the analog information in an implantable medical device to solve the above listed problems and shortcomings. The most common method is to digitize (i.e. change to digital format) an analog signal for storage or transmission. For example, U.S. Pat. No. 4,223,678 by Langer, et al., discloses the recording of EGM data prior to and following the detection of an arrhythmic event and subsequentially delivered shock. Pre-event analog data is converted by an analog to digital converter (ADC) and stored in auxiliary memory while post-event data is stored in main memory. Both memories are frozen upon storing of the single episodic event.
U.S. Pat. No. 4,407,288 by Langer, et al., discloses a two micro-processor based implantable defibrillator with ECG recording capability. Pre-event detection data is stored via a low speed processor into memory via direct memory access (DMA). Upon event detection, the data for a single event is frozen.
U.S. Pat. No. 4,625,730 by Fountain, et al., discloses the storage of ECG data via DMA to memory. One event may be stored for a total time duration of 10.24 seconds. Also, the '730 patent discloses a hand-held patient programmer used as an actuator to trigger the storage of an event.
U.S. Pat. No. 4,295,474 by Fishell discloses the recording of ECGs as in the '678 patent along with the recording of time and number of arrhythmic episodes since the previous office visit. The ECG is converted by a six bit ADC at 50 Hz for the storage and memory of one event for a total duration of 80 seconds. Ten seconds of continuously stored data is contained in a section of memory to enable the freezing and storage of 10 seconds of pre-event data and 70 seconds of post-event data.
The prior art listed above may be typically characterized by digitizing data with an ADC, the storage of a single event with limited duration (10-80 seconds), no provision for extended storage times or storage of multiple events/episodes, generally low fidelity signals, and extended telemetry transmission times to external peripherals.
To obtain extended storage capability (long storage time or multiple storage of events), digitized analog signals, such as an ECG, would require very large memory storage capability--140 million bits of memory per day--(see Chapter 1, Part 4 "The Third Decade of Cardiac Pacing"). Memory of that size would require a large number of integrated circuits and would not fit in a typically sized implantable pulse generator, would cause excessive current drain from the battery during operation and require nearly five hours to uplink to a peripheral utilizing a "state-of-the-art" telemetry system. Proposed solutions to the large amount of memory required include data reduction techniques such as blanking out all signals except for the signals indicative of the atrial and ventricular depolarization and repolarization (PQRST interval) which is then converted via standard techniques. This results in a reduction of the storage requirements of only approximately 50 percent. Additionally, compression techniques such as Coordinate-Reduction-Time-Encoding-System (CORTES) or Amplitude-Zone-Epoch-Time-Coding (AZTEC) may be used to achieve up to 10:1 data compression (see Data Compression--Techniques and Applications, Pg. 256-259, T. J. Lynch). However these techniques are not suitable for use in implantable medical devices because of the extensive processing power required to compress and store data real time.
Additionally, data stored in an implantable medical device's volatile memory can be erased or contaminated (flawed) by device failure, EMI, cautery or defibrillation procedures. Therefore, most implantable pulse generators have a power-on reset (POR) circuit that resets memory when power supply glitches occur. Any stored data is then erased and lost to the follow-up clinician.
Alternative methods proposed for the storage of analog signals in implantable pulse generators include magnetic bubble memories (see Chapter 3, Part 4 "The Third Decade of Cardiac Pacing") and Charge Coupled Devices (CCD). Present problems with bubble memories include the limited size of the memory (or alternatively, the large size of the integrated circuit), the difficulty in the read/write mechanism required and the complexity of the interface circuitry to the rest of the implantable pulse generator circuit (typically constructed of CMOS circuitry). Alternatively, in a CCD, the signal to be recorded is stored as a charge on an integrated capacitor. However analog information cannot be stored on a CCD for very long because the capacitor leakage rate on the CCD is too high to maintain accuracy for any significant length of time.
What is needed is a device that can electrically store analog information in an implantable medical device with reasonable precision, for long terms, with substantially reduced complexity, minimized current drain from the battery, greatly reduced memory requirements, increased telemetry transmission rates and the integration of signals marking specific function of the implanted device simultaneous with the recording of the analog signal. The system of the present invention provides for an efficient electronic recording and playback system for an implantable medical device which stores signal information in considerably less memory, with reduced complexity, and reduced current drain than that required for digital storage, and includes markers indicative of device function.